The Influence of the Tracer Injection-Acquisition Interval on the Distribution of Technetium-99m-Ethyl Cysteinate Dimer in the Feline Brain
T. Waelbers1; K. Peremans1; S. Vermeire1; A. Dobbeleir1; V.O. Boer2; H. de Leeuw2; M.A.D. Vente2; K. Piron1; M. Hesta1; I. Polis1
Introduction
The study of the regional cerebral blood flow, a measure for the regional brain activity, can be used in cats with behavioral problems or neurological problems like epilepsy. Single Photon Emission Computed Tomography (SPECT) can be used for this purpose. The lipophilic tracer 99mTechnetium-Ethyl Cysteinate Dimer (99mTc-ECD) can be injected in the awake animal. As it enters the brain cells this tracer is converted into a hydrophilic compound and is being trapped. Therefore, the acquisition can be performed afterwards under general anaesthesia, while the tracer distribution still represents the regional cerebral blood flow in the awake animal. In humans, this so-called fixed image is present for at least 120 minutes post injection.
Aim
The aim of this study was to look at the duration of the in vivo stability of 99mTc-ECD in the feline brain, which is important in view of the determination of an optimal tracer injection-acquisition interval.
Methods
For this purpose three serial acquisitions were performed under general anaesthesia, starting at 40 (T40), 60 (T60) and 85 (T85) minutes post tracer injection and total counts as well as perfusion indices were calculated. Semi-quantification, resulting in the calculation of perfusion indices (PIs), was performed by normalizing the regional counts to the counts of two reference regions. The use of the entire or total brain counts and the counts registered in the cerebellum resulted in the calculation of respectively PIT and PIC.
Statistics
One-way ANOVA was used with post hoc comparisons of the group means using the Tukey test. Significance was defined as P < 0,05.
Results
When T85 was compared with T40, total counts decreased, depending on the brain region, with 29% in the left thalamic region to 51% in the bulbus olfactorius and significant regional differences occurred in the perfusion indices as well, especially when the cerebellum was used as reference region.
Conclusion
This study shows that the clearance of 99mTc-ECD in the feline brain is region dependent. A distinct higher clearance of 99mTc-ECD in the cerebellum, compared to the other brain regions, renders the "entire brain" the preferred reference region for semi-quantification purposes. The results of this study suggest that, when 99mTc-ECD SPECT is used for studying the regional brain perfusion in cats, the acquisition should be started between 40 and 60 minutes post tracer injection.